It is well-known that serotonin (5-hydroxytriptamine, 5-HT) can act as a neurotransmitter as well as a neurohormone to regulate reproduction in marine invertebrates. Serotonin can prompt ovarian and embryonic development in crustaceans and trigger oocyte maturation and sequential spawning in bivalve mollusks. In this study, the effects of serotonin on the reproductive performance and quality of the offspring of sea cucumber (Apostichopus japonicus) treated with serotonin were examined. During hatchery broodstock conditioning, sea cucumbers were injected with serotonin at four different doses (20 μg/g, 50 μg/g, 100 μg/g, and 200 μg/g body weight) and four frequencies (every 5, 10, 15, and 20 days). Sea cucumbers injected with physiological saline were used as the control group, and untreated sea cucumbers were used as the blank group. At the end of the experiment, the spawning of sea cucumbers was triggered using a combination of desiccation, running water, and thermal shock. The results revealed that the spawning rate and fecundity of female sea cucumbers injected with serotonin were higher than those of the control and blank groups. The female sea cucumbers injected with serotonin at dose of 50 μg/g body weight every 10 days showed the highest spawning rate and fecundity of (33.3±1.9)% and (637.0±111.1)×104 cells per individual, respectively. The egg diameter, fertilization rate of eggs, hatching rate of larvae, body length of late auricularia larvae, and settlement rate of larvae in the 50 μg/g body weight every 10 days group and blank group were evaluated further. The egg diameter, fertilization rate, hatching rate, larval body length, and settlement rate of the 50 μg/g body weight every 10 days group were (165.0±3.5) μm, (93.0±1.5)%, (82.7±1.2)%, (856.0±24.8) μm, and (24.0±1.9)%, respectively. No significant differences were observed in these parameters between this group and blank group. These results indicate that serotonin, as a neurotransmitter, prompts ovarian development in sea cucumbers, and injecting serotonin during broodstock conditions can improve the reproductive performance of female sea cucumbers.